1. Field of the Invention
The present invention relates to a predistorter having an automatic gain control (AGC) circuit and method therefor and, in particular, to a simplified predistorter circuit which lacks a subtraction circuit typically found in a conventional predistorter, and includes an automatic gain controller (AGC) and method therefor.
2. Description of the Related Art
A predistorter is a circuit that minimizes an intermodulation distortion of a main amplifier. This is accomplished by estimating the intermodulation distortion in the main amplifier and adding the predistorted signal to the input signal of the main amplifier.
Referring to FIG. 1, a conventional predistorter comprises a harmonic generator 10 used for the predistortion, and a subtraction circuit 40, which removes all but the spurious components among the signals being generated in the harmonic generator 10, wherein the spurious components contribute to the linearization of the main amplifier. The subtraction circuit 40 aids in canceling the main carrier components being amplified in the harmonic generator 10.
The subtraction circuit 40 receives the input signal via divider 5 and comprises an attenuator 50, a phase shifter 60, a combiner 52 and a divider 51. The gain in the attenuator 50 and the phase shifter 60 is the same, and the phase is inverse, as compared to the gain and phase of the harmonic generator 10.
A first variable attenuator 20 and a variable phase shifter 30 are used to optimize the resulting signal after canceling the main carrier components, thereby minimizing the intermodulation distortion of the main amplifier 80.
However, practically speaking, the subtraction circuit 40 can not completely cancel the main carrier components. In addition, the subtraction is not performed when the frequency band is narrow and the frequency is changed.
As such, a problem occurs in that the gain of amplifier becomes unstable overall when the first variable attenuator 20 and variable phase shifter 30 are operated so as to minimize the intermodulation distortion of the main amplifier. Here, the gain of the overall amplifier is calculated as a gain derived from the predistorter and the main amplifier.
The gain of amplifier also becomes unstable overall because the spurious components power is that of a specific rate over the total signal.